As wind energy installations become increasingly widely used, they are increasingly also used to regulate and protect the power transmission grid. The range of objects of the wind energy installations is therefore no longer restricted just to simple production of electrical energy, but is extended to the provision of so-called system services, which the grid operators require in order to maintain stable grid operation. In addition to feeding reactive power, these system services also include the provision of a real-power reserve (so-called regulation reserve). If the power consumption in a grid system exceeds the power production, then the grid frequency falls; conversely, the grid frequency rises when the power fed in exceeds the power consumed. Since frequency changes such as these are undesirable, their occurrence is reacted to by feeding in additional power (if the frequency is low) or reducing the power fed in (if the frequency is high). In order to allow a quick reaction here, even at short notice, appropriate power-station capacities must continuously be kept available (so-called second reserve). This is supplemented by power-station capacities which can be brought on line quickly when required (so-called minutes reserve). The setting up and operation of these power stations is very expensive (particularly in the second reserve area). Particularly in the case of grid systems where a relatively large number of wind energy installations feed power in, in addition to conventional power stations, there is therefore also a requirement for the wind energy installations to be used to provide the regulation reserve. One particular difficulty in this case is that, although wind energy installations can reduce their power output (DE 100 22 974 A1), it is not easily possible to increase their power—in contrast to conventional power stations—since the maximum available power is restricted by the available wind, and therefore fluctuates stochastically.
In order nevertheless to also allow wind energy installations to be used to provide regulation power, various approaches have become known in the prior art. In one widely used approach, use is made of the fact that wind energy installations are normally combined to form wind farms. Here, a wind energy installation (reference installation) is operated with the full power provided by the respective wind conditions, and the electrical power produced by it is used as a reference for the other wind energy installations in the farm. Depending on the desired power reserve, these other installations are then operated on a restricted basis, thus allowing additional power to be mobilized quickly when required, by removing the restriction. This quite simple concept in principle works, but has the disadvantage that discrepancies in the maximum power capability between the individual wind energy installations in the wind farm are not taken into account by the conditions found at the reference installation. There is therefore no guarantee that the desired regulation power will also actually be available, and in fact this is random.
It is also known for an anemometer to be used to measure the actually prevailing wind speed, and for the power available from this wind to be determined on the basis of a characteristic determined for the respective wind energy installation (DE 10 2007 036 446 A1). This value represents the available power, from which a certain amount is subtracted as the power to be kept in reserve (regulation power). The value which then results is set as a modified power nominal value by the operation controller for the wind energy installation. One advantage of the method is high transparency in its own right, but it has the disadvantage that errors in the measurement of the wind speed are passed through in their entirety to the result. This has a particularly serious effect, because the measurement of the wind speed at the wind energy installation by means of an anemometer arranged on the nacelle is known to be imprecise. Because of this, instabilities may occur, particularly when the discrepancies accumulate over a relatively long time period. This applies in particular to the secondary regulation, since this relates to a longer time period than the rapid primary regulation (also referred to as the second reserve).
It is also known for the kinetic energy stored in the rotor to be used for rapid mobilization of additional power. This is done by producing more electrical power than is provided per se by the rotor in the given wind conditions, by increasing the generator torque. This admittedly makes it possible to mobilize additional power very quickly, virtually independently of the currently prevailing wind conditions, but this is at the expense of a falling rotor rotation speed, and can therefore be carried out only over a short time period. The use of this method is therefore restricted to the primary regulation area.
It is also known for wind energy installations not to be operated with values defined per se by the operation controller for the respective operating point, in particular for the pitch angle, but in this case to reduce them by a safety margin (Prillwitz, F. et al.: Primärregelung mit Windkraftanlagen [Primary regulation with wind power installations], ETG-Workshop “Neue dezentrale Versorgungsstrukturen” [New decentralized supply structures], 19-20 Feb. 2003, Frankfurt am Main). With respect to the pitch angle, this means that the installation is operated with a pitch angle reduced by a fixed amount (safety margin). Since the installation therefore does not run with the optimum pitch angle, this results in a power reserve which can be mobilized by setting the pitch angle to the optimum value. One disadvantage of the method is that, because the pitch angle is reduced statically by a fixed amount, different power reserves result, depending on the installation operating point. While this may still be acceptable for the primary regulation in the seconds area (since the operating point normally does not vary or varies only slightly over a few seconds), a response such as this is significantly disadvantageous for the secondary regulation, which relates to a longer time period.